After 2 years of using sidux/aptosid, I recently went through a complete reinstallation of my thinkpad x61s. I installed aptosid-2011-01-geras-amd64 and the backlight behaves in an unexpected way.

While the AC adaptor is plugged in and backlight is set to the max, it works as intended, but if I drop backlight brightness one step, either through the dedicated keys or the /sys/class/backlight/acpi_video0/ files, or if I unplug the AC then the backlight turns off and the screen goes black.

I also tried setting kde power management to use the same power profile when unplugged and set brightness to max in all power profiles but it as no effect.

Interestingly, /sys/class/backlight/acpi_video0/max_brightness has a value of 15. Unplugging the power cord doesn't change /sys/class/backlight/acpi_video0/actual_brightness value which stays at 15. When plugged, changing the value to 14 makes the screen go black.